An automatic transmission that incorporates an electromagnetic valve and adjusts variables relating to a gear shift operation, such as a gear, a hydraulic pressure level, a time constant or timing in the gear shift operation, and the like, by means of an ECU (Electronic Control Unit) by receiving an electrical signal from the outside has been put into use. Such an ECU allows reliable and prompt transition of an operation state of the automatic transmission to various states. In addition, as the ECU incorporates a CPU (Central Processing Unit), control using program can be carried out. Accordingly, if an operation state of the automatic transmission is finely set by modifying a program or various constants, optimal performance of the automatic transmission can be obtained in line with a running state of a vehicle or a state of load onto an engine. Here, the running state of the vehicle refers to a vehicle speed, a steering operation, frequency and a level of acceleration and deceleration, a road surface state, and the like, while the state of load onto the engine refers to an engine speed, a throttle position, an accelerator pedal pressing degree, torque of input and output shafts of an engine or an automatic transmission, and the like.
The above kind of automatic transmission has a multi-gear transmission mechanism having a plurality of power transmission paths or a continuously variable transmission mechanism, e.g., of a belt type, and is configured, for example, such that the power transmission paths are automatically selected or diameters of pulleys carrying a belt is automatically changed based on the accelerator pedal pressing degree and the vehicle speed, and thereby the speed change ratio is automatically changed.
For implementing the above automatic transmission, a mechanism that can keep a vehicle in a stopped state while an engine is operating, and can also transmit a drive power of the engine to a transmission mechanism (i.e., a mechanism allowing slipping between input and output shafts) must be arranged between the engine and the transmission mechanism. Fluid couplings are a kind of such mechanisms, and torque converters having a torque amplifying function are presently used in many cases.
For the torque converters, the following technology has been known. A lockup clutch that can directly couple input and output shafts of the torque converter together is controlled to perform feedback control (slip control) so that a coupling force of the lockup clutch may attain a predetermined state according to a difference between a pump rotation speed on an input side (corresponding to an engine revolution speed) and a turbine rotation speed on an output side. Based on a learned value obtained thereby, feedforward control is appropriately performed on the slip state of the torque converter so that vibrations and noises (NV: Noise & Vibration) are prevented, and a vehicle starting performance is improved.
By the above highly developed electronic control, the power transmission allocation of mechanical power transmission through the lockup clutch and power transmission through the torque converter is finely control according to the driving state, and thereby transmission efficiency is significantly increased.
Thus, this lockup clutch is controlled based on the driving state of the vehicle such as a load, revolution speed and the like. For example, a low-load and high-speed region is set to a lock-up region, a high-load and low-speed region is set to a converter region, and a low-load and a middle-speed region is set to a slip region. In the lock-up region, input and output elements of the hydrodynamic power transmission device are completely coupled together to improve fuel consumption. In the converter region, the input and output elements of the hydrodynamic power transmission device are completely released from each other to increase the torque by a torque amplifying function of the torque converter. In the slip region, the input and output elements of the hydrodynamic power transmission device are partially coupled to improve the fuel consumption and to absorb shocks and vibrations.
When the driving state of the vehicle changes from the converter region to the slip region, the coupling force of the lockup clutch is usually controlled (i.e., the above slip control is performed) so that a slippage between the input and output elements of the hydrodynamic power transmission device may converge to a predetermined target slippage. In this operation, when a long time (long converging time) is required for converging the actual slippage to the target slippage, this lowers an effect of improving the fuel consumption. Conversely, when the converging time is short, the engine revolution speed rapidly lowers to cause deterioration of drivability and drive feeling. Accordingly, it has been known to perform learning correction on a control quantity of the coupling force of the lockup clutch (e.g., a duty ratio applied to a solenoid pipe arranged in a hydraulic circuit) according to individual differences, variations, secular changes and the like of the lockup clutch, the solenoid valve and others so that the converging time may become equal to the predetermined target time keeping the above problem unnoticeable (i.e., the slippage may converge to the predetermined target slippage at a target time).
Japanese Patent Laying-Open No. 2004-150548 has disclosed a slip control device of a lockup clutch that can increase a frequency of learning as far as possible in slip control of a lockup clutch. The slip control device of the lockup clutch includes a determining unit determining whether a driving state of a vehicle belongs to a lock-up region where input and output elements of a hydrodynamic power transmission device are completely coupled together, a slip region where the input and output elements are partially coupled together or a converter region where the input and output elements are completely released, and a slip controller controlling a coupling force of the lockup clutch to converge a slippage between the input and output elements of the hydrodynamic power transmission device to a predetermined target slippage when the determining unit determines that the driving state of the vehicle changed from the converter region to the slip region. This slip control device of the lockup clutch further includes a setting unit setting a target value of the slippage at a predetermined point in time before the slip control by the slip controller converges the slippage between the input and output elements of the hydrodynamic power transmission device to the target slippage, a correcting unit performing correction according to the target value set by the setting unit and an actual value of the slippage at the predetermined point in time, and thereby correcting a control quantity of the coupling force of the lockup clutch to attain the actual value equal to the target value, and a learning unit causing the slip controller to perform next slip control, using the control quantity corrected by the correcting unit.
This slip control device of the lockup clutch sets in advance the target value of the slippage attained between the input and output elements of the hydrodynamic power transmission device at the predetermined point in time before the slip control converges the above slippage to the target slippage, and the learning correction is performed on the control quantity of the coupling force of the lockup clutch according to the target value and the actual value at the above predetermined point in time. Therefore, data for performing the learning correction on the slip control can be collected or the learning correction can be performed before the slippage actually converges to the target slippage, without waiting for the completion of the slip control. Consequently, the frequency of the learning correction increases regardless of the state and progress of subsequent slip control (i.e., without being affected whether the slip control is fully completed or is interrupted before the full completion), and this removes the influence by individual differences, variations, secular changes and the like of the lockup clutch so that improvement of the accuracy of the slip control is promoted.
As described above, there has recently been a strong tendency to enlarge further the lock-up region, and the learning frequency in the lock-up region tends to decrease. In Japanese Patent Laying-Open No. 2004-150548 already described, when the slip control does not start, the learning control will be performed before the slippage (i.e., the result of the slip control) converges to the target slippage, i.e., without waiting for the completion of the slip control. However, when the slip region is narrow, the slip learning control starts at a low frequency, and the effect similar to that in Japanese Patent Laying-Open No. 2004-150548 cannot be expected.